一个计算器的问题

做一个计算器,但遇到问题
我是初学者,请达人帮助下:
import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
class Minicalc{
public void main(String[] args)
{
  JFrame frame=new JFrame("简易计算器");
  Container pane=frame.getContentPane();
  JTextField firstnumber=new JTextField(10);
        JTextField secondnumber=new JTextField(10);
        JTextField result=new JTextField(10);
        JButton addButton=new JButton("+");
        JButton subButton=new JButton("-");
        JButton mulButton=new JButton("*");
        JButton divButton=new JButton("/");
        pane.setLayout(new GridLayout(5,2));
        pane.add(new JLabel("Enter a number"));
        pane.add(firstnumber);
        pane.add(new JLabel("Enter a number"));
        pane.add(secondnumber);     
        pane.add(new JLabel("Result"));
        pane.add(result);
        pane.add(addButton);
        pane.add(subButton);
        pane.add(mulButton);
        pane.add(divButton);
        
        DoMath listener=new DoMath(firstnumber,secondnumber,result);
        addButton.addActionListener(listener);
        subButton.addActionListener(listener);
        mulButton.addActionListener(listener);
        divButton.addActionListener(listener);
        frame.pack();
        frame.show();
        
        
        
}
class DoMath implements ActionListener
{
       private JTextField inputone,inputtwo,output;
      // char F[]=(e.getActionCommand())toCharArray();
   
       DoMath(JTextField first,JTextField second,JTextField result)
          {
             inputone=first;
             inputtwo=second;
             output=result;
          }
        public void actionPerformed(ActionEvent e)
          {
              double first,second;
              first=Double.parseDouble(inputone.getText().trim());
              second=Double.parseDouble(inputtwo.getText().trim());  
              if (e.getActionCommand().equals("+")) 
                      output.setText(String.valueOf(first+second));
              else if(e.getActionCommand().equals("-")) 
                    {output.setText(String.valueOf(first-second));}
              
              else if(e.getActionCommand().equals("*")) 
                          {output.setText(String.valueOf(first*second)) ;}
              else
                 {output.setText(String.valueOf(first/second));}
                    
            }
    }
 }